Currently, it is a major tendency of the development of steel and iron materials, to adopt thin steel sheet of higher strength to make steel structures thinner and lighter. At the same time, those hot rolled or cold rolled weathering steel sheets which are widely applied in transportation industries, evolve in the direction of higher strength and lower alloy content, so as to meet the demands of the terminal customers on weight reduction, energy conservation, lower cost. Since 1930s when United States Steel Corporation first developed low-alloy Corten steel containing Cu with erosion resistance and high tensile strength, a series of Corten A steel with high content of P and Cu plus Cr, Ni, and a series of Corten B steel with alloying Cr, Mn, Cu, had been formed. Subsequently, a series of weathering steels treated by rare earth were developed in China. With respect to improving the strength of the weathering steel, the major measures are solid solution strengthening, precipitation strengthening, transformation strengthening, etc., wherein the latter two strengthening methods are mainly employed to improve the strength of weathering steel of super strength.
China patent of Publication No. CN1884608A, published on Dec. 27, 2006 and titled “method of producing 700 MPa level V-N micro-alloying high-strength atmosphere corrosion resistant steel based on continuous casting and rolling process of thin slab”, discloses a method of producing high-strength atmosphere corrosion resistant steel. Depending on the characteristics of continuous casting and rolling process and metallurgical compositions of thin slab, the method adopts processes of electric furnace or converter smelting, refining, thin slab continuous casting, entering the roller hearth heating or soaking furnace directly after casting blanks solidify, hot rolling, laminar cooling and coiling, wherein, the ranges (wt %) of chemical compositions of the molten steel are: C: less than or equal to 0.08%; Si: 0.25˜0.75%; Mn: 0.8˜2.0%; P: 0.070˜0.150%; S: less than or equal to 0.040%; Cu: 0.25˜0.60%; Cr: 0.30˜1.25 wt %; Ni: less than or equal to 0.65%; V: 0.05˜0.20%; N: 0.015˜0.030%.
US patent of Publication No. U.S. Pat. No. 6,056,833, published on May 2, 2000 and titled “thermomechanically controlled processed high strength weathering steel with low yield/tensile ratio” relates to a weathering steel sheet with a low yield/tensile ratio that has a minimum yield strength of 70-75 ksi and a yield/tensile ratio less than or equal to about 0.85. The chemical compositions of the weathering steel sheet consists (wt %) of C: 0.08˜0.12%; Mn: 0.80˜1.35%; Si: 0.30˜0.65%; Mo: 0.08˜0.35%; V: 0.06˜0.14%; Cu: 0.20˜0.40%; Ni: 0.50%; Cr: 0.30˜0.70%; P: 0.010˜0.020%; Nb: less than or equal to 0.04%; Ti: less than or equal to 0.02%; S: less than or equal to 0.01%; the remainders thereof being Fe and other unavoidable impurities.
Korean patent of Publication No. KR431839, published on May 20, 2004, and titled “method of producing cold rolled weathering steel sheet”, discloses a method of producing the cold rolled atmosphere corrosion resistant steel sheet, wherein, the chemical compositions of the steel sheet consist of C: 0.06˜0.08 wt %; Si: 0.17˜0.24 wt %; Mn: 0.9˜1.10 wt %; P: less than or equal to 0.020 wt %; S: less than or equal to 0.010 wt %; Cu: 0.20˜0.30 wt %; Ni: 0.20˜0.30 wt %; H: less than or equal to 2.5 ppm, the remainders thereof being Fe and other unavoidable impurities. The steel sheet has a tensile strength of more than or equal to 45 kgf/mm2, a yield strength of more than or equal to 32 kgf/mm2, and an elongation rate of more than or equal to 22%.
The aforementioned first and second patents use hot rolling processes to produce the weathering steel sheet, and due to the limitation with respect to the thickness of steel sheet in hot rolling units, the weathering steel sheet produced in hot rolling process has often a large thickness, and further, the limit thickness of available the hot rolling steel sheet increases with the improvement of the strength thereof. Besides, the shape and surface quality of the hot rolling steel sheet is still poor than that of cold rolling steel sheet. Although the above third patent adopts cold rolling process to produce the weathering steel, the steel sheet presents a low strength (the yield strength is only 300 MPa), which cannot be widely used in the production of high strength steel structures.